1. Field of the Invention
The present invention relates to a rangefinder for automatically measuring a distance to an object, and more particularly to a semiconductor light position detector used in an automatic focusing device of a camera that automatically brings an object to be photographed into focus.
2. Description of the Prior Art
In order to automatically measure a distance to an object, several methods have already been proposed and put into practical use. Among them, a so-called light projection method has been recently used for the automatic focusing device or the like of a still camera or video camera, because it assures stable measurement of distance to a low contrast object and a dark object under poor illuminance. Specifically, a light beam such as an infrared light beam is projected onto an object, and a reflected light from the object is received as a light spot by a semiconductor position detector to determine the distance to the object.
An example of such a conventional light projection method is disclosed in Japanese Patent Application Laid-Open No. 60-87576, in which an infrared LED produces a light beam projected to an object for measuring a distance to such an object. A reflected light beam from the object is received by a semiconductor light position detector (hereinafter referred to as light position detector). The distance to the object is determined from a position of a light spot (more specifically centroid of luminance of the spot) formed by the reflected light beam on the surface of the detector.
A known device usually called a position sensitive device (PSD) is used as the light position detector for this method. The PSD consists essentially of a PIN photodiode having a high resistance layer on its surface. The structure of the PSD is described in detail in "Semiconductor Position Detector and its Application," in Electronic Material, Japan, Feb. 1980, pp. 119-124.
The aforementioned infrared LED projects a light beam having a light intensity distribution depending on the shape of its light-emitting portion onto an object to be photographed. A light spot also depending on the shape of the light-emitting portion is formed on the light-receiving surface of the light position detector. Referring to FIGS. 4(a)-(d), there are shown the optical position detector and the light spot produced when the infrared LED has a semispherical or circular light-emitting portion. A light position detector 9 has an effective light-receiving surface 10, and two aluminum electrodes 11 and 12 for taking out produced photocurrent. Ideally, as shown in FIG. 4(a), a light spot 13 reflecting the shape of the light-emitting portion of the infrared LED (not shown) is formed, and a light intensity distribution shown in FIG. 4(b) is obtained. In reality, however, the light spot formed on the light-receiving surface 10 may include a flare component 15 beside a main component 14 reflecting the shape of the light-emitting portion of the infrared LED, as shown in FIG. 4(c). The projected light from the LED is reflected from the inner wall of the housing can and the edge surface of the cover glass of the light projector, producing the flare component 15 usually assuming a circular distribution. As already mentioned, the rangefinder determines a distance from the position of the light spot. Accordingly, when the flare component 15 is large, for example when another object having a high reflectivity exists near the object of interest, the light intensity distribution on the light position detector has two peaks as shown in FIG. 4(d). In this case, the distance is determined from the spurious centroid of luminance of the spot, which is found from the weighted average of the main component 14 and the flare component 15. Thus, the measured distance involves an error.
Also in the arrangement described above, if the distance to the object is measured under a high illuminance, then a large amount of unwanted light enters the light position detector in addition to the infrared light used for measurement. This unwanted light produces unwanted photocurrent. As a result, a large quantity of shot noise is developed in proportion to the square root of the amount of the photocurrent. This is due to the fact that the light position detector responds considerably keenly to the wavelengths of the sunlight or of the light emitted by an illuminating halogen lamp. FIG. 5 shows a typical example 16 of the spectral sensitivity characteristic of the light position detector, the spectral characteristic 17 of the sunlight, and the spectral characteristic 18 of a halogen lamp at a temperature of 2200 K. Usually, the infrared light for measurement is modulated with a certain frequency f.sub.o to be discriminated from the unwanted light. The arithmetic unit for determining the distance incorporates a frequency-separating filter and a synchronous detector circuit to extract the component arising from the infrared light for measurement. However, when the amount of the unwanted light incident on the detector is quite large, those noise components of the unwanted light which approximate the certain frequency f.sub.o deteriorate the accuracy of the measurement. As a result, the hunting of the taking lens, a defocusing, or other malfunction will take place.